Construction of a 70cm band
Mini Kits EME221-70CM pre-a
mp kit - November 2020
The Mini Kits EME221-70CM kit is a high-performance masthead mount receiver pre-amplifier specifically designed for the 70cm band (430 to 450MHz). Please see the specifications below. I embarked on this project because I had experienced some success working through the ISS repeater on several occasions. However, I felt that the receiver sensitivity of the Yaesu FT-736R 70cm module was not as good as I would have liked, and a masthead-mounted pre-amplifier seemed like an ideal solution. I considered several fully constructed commercial pre-amps, but ultimately chose the Mini Kits EME221-70CM pre-amp kit based on its advertised performance specifications and significantly more competitive price.
The idea of constructing a quality kit also appealed to me, although it was a bit daunting as I had limited experience with SMD (Surface Mount Device) construction. However, I believed it would be a good opportunity to gain some experience.
The kit notes do state, 'Do not construct this Kit unless you are very experienced with surface mount
to 450MHz ( BW 20MHz @ 1dB )
>35dB @ 145MHz, >60dB @ 900MHz
+29dB Max, 20dB Min @ 435MHz
( Typically <0.9dB @ 435MHz )
@ 435MHz ( Bypass Mode )
CW ( Un-Sequenced ) 100W PEP ( Sequenced )
( 100mW ) To activate the RF sensing circuit
to 15vdc ( +12vdc @ 220mA Recommended )
L x 84mm W x 17mm H ( with TNC connectors )
Photo 1 Completed
assembly of the PCB
The construction notes were excellent, and the construction process generally went smoothly, except for accidentally dropping a small SMD resistor on the floor. It is still there and will never be seen again. Once the replacement resistor was sourced, the board construction was completed.
The pre-amp includes RF sensing circuitry and relays that can operate up to 50W CW/FM and 100W PEP SSB. However, in this installation, the DC power and switching will be injected into the coaxial cable via a Bias Tee and controlled by the Yaesu FT-736R STBY Jack.
The tuning and performance testing were completed using a NanoVNA in LOGMAG mode, which revealed that the amplifier was producing 28.77dB gain at the target frequency of 436MHz. To avoid overloading the amplifier, a 20dB attenuator was required between the CH0 port and the input to the amplifier when using the NanoVNA.
Photo 2 Amplifier
gain measurements with a NanoVNA.
The NanoVNA also showed that the 20MHz bandwidth (covering the entire 70cm amateur band) was within 1dB of the peaked frequency of 437MHz. The gain adjustment range was also measured from a maximum of +28.77dB to approximately +19dB, which is consistent with the specifications. The alignment of the amplifier involves tuning the C2 trimmer for maximum gain at the desired frequency.
While on the bench, several satellite beacons were monitored both with and without the amplifier switched on. In one instance, the satellite beacon was barely discernable in the noise but improved to an S6 level with the amplifier. While this is not a completely scientific measurement since satellite signals are highly variable, it provided a dramatic demonstration of the expected performance.
Overall, the kit is fantastic, performs as specified, and is designed in Australia. It also has an appealing aesthetic and was enjoyable to construct.
Video of the construction
and testing of a masthead pre-amplifier kit designed for the 70cm band
(430 to 450MHz). The video includes an overview of the kit, examples
of my first attempts at the installation of surface mound devices and
final performance testing.
Video is of a NanoVNA being used to measure the gain of a 70cm band pre-amplifier including the general set up and VNA calibration. This video is an exert from an earlier video detailing the full construction video of a Mini-Kits 70cm band
of on air testing using the amateur satellite RS-44 beacon.